This invention relates to an apparatus and method for testing human ability to control the focus of an eye. The invention has particular utility in connection with the prescription of intraocular of multifocal contact lenses.
The human eye focuses on an object by flexing itself so as to bring the retina into the image plane of an object being viewed. The position of the image plane in turn depends upon the distance of the object being viewed and the focal length of the lens of the eye. As the eye ages, the focal length of the lens changes, and the image plane moves to positions which cannot be reached by the retina. Prescription lenses compensate by shifting the position of the image plane. As the aging process continues, the eye muscles lose their flexing ability and eventually deteriorate to the point where they cannot accommodate the range of image plane positions associated with near and far objects. It then becomes necessary to provide multifocal length corrective lenses. Whether a multifocal lens is of the conventional type or the contact type, the focal length varies in an angular pattern expected to correspond to the variation of object distances.
The person who wears a conventional multifocal corrective lens tilts his head so as to position the lens at an angle which places the correct focal length region thereof along the optical axis between the eye and the object. The eye rotates in the opposite direction and then flexes that amount which will bring the retina into the image plane of the object.
A different situation obtains for the eye which is fitted with a multifocal contact lens. Here the lens moves with the eye, and creates situations wherein different images present in the instantaneous field of view may be focussed at different image planes. In this situation the eye cannot be flexed so as to bring both images into simultaneous perfect focus. Consequently, the brain must select one image or the other and control the eye muscles to bring that image into focus. This requires a special skill which is not well developed in some persons. Heretofore there has been no test for this skill. Indeed there has been no appreciation of the need to perform such a test. The practitioner has measured and tested the eye with a view toward obtaining the correct physical fit and the proper focal length range. Cognitive skills have been neglected, and therefore it has sometimes happened that the patient has developed a physiological reaction in the form of headaches or blurred vision. This has occurred after the lens has been worn for a period of time, and it cannot be corrected by a change of prescription.
It has been found in accordance with the practice of this invention that it is practical to test the cognitive skill related to focus control. It is believed that the results of such a test provide an indication of adaptability to contact lenses. It is also believed that the focus control skill may be impaired by a variety of physiological conditions and that an effective test of that particular skill may provide a valuable diagnostic tool.